This invention relates to internal combustion engines, and more particularly to small internal combustion engines of the type used to power lawnmowers, snow blowers, generators and the like.
Several types of ignition systems are known for small internal combustion engines. One such type is the capacitive-discharge ignition system, wherein a charge capacitor is charged from a current source such as a charge winding, and is discharged in response to the gating on of a thyristor in series with both the charge capacitor and with the primary winding of an output coil. The gating on of the thyristor controls discharge of the capacitor through the output coil, which triggers ignition firing.
Since the gating of the thyristor in effect controls the timing of ignition firing, control of thyristor gating may be used to retard ignition timing when the engine runs at low speeds upon starting, and to advance ignition timing when the engine runs at higher speeds. Unfortunately, typical prior art gating control techniques are complicated and expensive in that they often require additional trigger coils or a number of semiconductor switches. Although capacitive-discharge ignition systems having continuous timing advance are desirable in some applications, an inexpensive step advance system is suitable for many types of engines.